Method of continuously measuring the shear viscosity of a product paste

ABSTRACT

The present invention relates to a method of continuously determining the shear viscosity (η) of a product paste to be delivered to a spray nozzle for spray-drying applications wherein the continuous determination of the shear viscosity (η) of the product paste is carried out in a bypass to the product paste stream to the spray nozzle.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a National Stage of International ApplicationNo. PCT/EP2015/081096, filed on Dec. 22, 2015, which claims priority toEuropean Patent Application No. 14200752.5, filed on Dec. 31, 2014, theentire contents of which are being incorporated herein by reference.

The present invention is directed to a method of continuously measuringthe shear viscosity of a liquid product, said shear viscosity being in arange of 20 to 1000 mPa·s and having a shear rate greater than 1000 s⁻¹and a Reynolds number smaller than 2300.

Optimizing the operating conditions of product processing is the objectof intensive research in the industry, for example in production linesprocessing emulsions, suspensions and dispersions in view for example ofevaporation or spray-drying processes. Determination of the best economyoperating and control of such conditions is important in order to carryout processes in a cheaper and more environmentally sustainable way andto improve the product quality. It is therefore an objective of thepresent invention to provide methods enabling the skilled person toadapt the processing conditions to the characteristics of the processedproduct.

The manufacturing of food powders is realized to a great extent by meansof spray drying. This process converts emulsions, suspensions anddispersions into powder. Spray nozzles create droplets, which are driedin hot air by evaporating water. The final powder quality, the finalpowder texture, the dryer process design, the drying efficiency, thewalls fouling behaviour, the operational safety, to name only a fewcharacteristics, are directly linked to the spray quality and thus theatomization process.

Known spray drying processes use atomization nozzles with fixedgeometries which cannot be adjusted inline to the process and productconditions during start-up, manufacturing operation and shut-down.Instead operators change the nozzle geometries prior to the productioncycle without the possibility to cover all the manufacturing situations.Such nozzles are chosen according to water tables. The manufacturing offood powders happens at significantly higher viscosities compared towater. Typical spray viscosities are within in a range comprised between1 to 300 mPa·s. There is no known nozzle apparatus capable to competewith such a wide range.

As an example, for dairy emulsions at concentrate total solids above50%, the concentrate viscosity increases in an exponential slope withfurther increase of total solids. This fact causes problems tospray-drying, if the concentrate viscosity exceeds a design limit of theatomizer nozzles. The design limit is described by means of an atomizerair-core break-down, which stops the creation of droplets and thus stopsefficient spray-drying and agglomeration of powders with a requiredtexture. Using prior art spray nozzle apparatus, air-core break downswithin atomizer nozzles cannot be determined visually, thus there iscurrently no means to operate the spray-drying process at its best pointwithout facing issues, such as powder blockages in cones and cyclones,wall fouling or atomizer beard formation, to name just a few issues.

Since the product and process conditions change from start-up toshut-down of the process the quality of the product achieved varies andproduct build-up can happen on the nozzle itself and on the walls of thespray-drying equipment, in particular on the walls of the dryingchamber, in cones of spray-dryers and cyclones, but also in theconveying ducts between the process units.

It is a first objective of the present invention to overcome theproblems identified with prior art equipment and methods and to enableto operate a product paste processing, such as for example an evaporatoror a spray-drying equipment at its best point and in the most economicalway, which involves to be able to process material having the highestpossible total solids content and, in the case of spray-drying, toobtain a dry powder having the maximum total solid content possibleduring atomization, without exceeding the design limit of the atomizersnozzles, which is triggered by the air-core break-down.

It is an object of the present invention to obtain a method of measuringthe shear viscosity of a product in line during the production process,such as to enable control of the processing conditions and optimizationof the process. In the case of a product to be spray-dried, the spraydroplet size of a spray nozzle apparatus is controlled, which allowscontrolling of the working process and optimization of the spray-dryingprocess. This is particularly useful to achieve a target spray dropletsize distribution defined by the Sauter diameter and to keep a targetdroplet size distribution constant even with changing product ormaterial properties and changing process conditions.

This object is achieved by a method of continuously determining theshear viscosity (η) of a product paste in a processing line, wherein thecontinuous determination of the shear viscosity (η) of the product pasteis carried out in a bypass to the product paste stream, wherein thebypass comprises a pump, a flow meter, a differential pressure tube anda pulsation damper and wherein the shear viscosity is in a range of 20to 1000 mPa·s, the shear rate is greater than 1000 s⁻¹ and the Reynoldsnumber is smaller than 2300.

The shear viscosity is used as input parameter to control the processparameters.

In an embodiment, the product is to be processed in a spray-dryingequipment or an evaporator. The shear viscosity is used as inputparameter to control the evaporator or the spray nozzle. It allowsinline control of the evaporator or the spray nozzle. Thus, in the caseof a spray nozzle it allows inline control of the spray droplet size,via a stability criterion composed of the spray mass flow rate Qm, thespray pressure P the product density (ρ) and the product viscosity (η).In the case of an evaporator, the liquid film thickness can be maximizedwithout liquid film break-up

Furthermore, the control of the spray nozzle thanks to in linedetermination of the shear viscosity enables to achieve a consistentpowder agglomeration in the product during a production cycleindependent of the total amount of solid particles (TS) or independentof mass flow rate fluctuations. By this method, a process automation canbe achieved through improved and simplified reproducibility andreliability of product properties for different spray-dryer types. Acompetitive production control is achieved via advanced design of finalpowder properties like powder moisture, tap density, final agglomeratesize and agglomerate stability. Due to the automation the productioneconomy and process efficiency (best-point operation) is also enhanced.

In a preferred embodiment the shear viscosity (η) of the product pasteis determined by the following steps:

a) providing a constant feed-flow-rate of the product paste at laminarflow conditions;

b) determining the mass flow of the product paste;

c) delivering the product paste to a pressure-drop-meter and determiningthe differential pressure;

d) calculating the shear viscosity (η) of the product paste on the basisof the laminar mass flow and the product density determined in step b),as well as the pressure drop determined in step c).

More preferably, the calculation in step d) considers also thebypass-mass-flow-rate.

This method enables inline recording of product shear viscosities e.g.of coffee and milk products before atomization with its specific productcharacteristics such as highly viscous (>100 mPa·s) and shear-thinningflow behaviour (determination of 2^(nd) Newtonian plateau viscosity (η).The inline shear viscosity information is necessary to operate acontrollable evaporator or spray-nozzle inline in order to determine thebest point configuration of the evaporator or atomizer and warn in caseof design limit achieved. The inline differential pressure drop methodallows a calibration of the shear viscosity for Newtonian and inparticular Non-Newtonian shear-thinning fluids based on laboratoryrheometers.

Other techniques to measure the shear viscosity are eitherunderestimating or overestimating the predefined product shearviscosities of dairy and nutrition products (via laboratory rheometer).In particular for shear-thinning fluids, the frequency-based measuringtechnique, the Coriolis forced measuring method and thequartz-viscosimetry method do not give the possibility to determine the2nd Newtonian plateau viscosity of shear-thinning fluids due to the lackof information concerning the applied flow field of the method (and thusunknown shear rates).

Thus, inline recording of the so called second Newtonian plateauviscosity of Non-Newtonian food fluids is possible with the differentialpressure drop method and thus allows calibration with predefined productshear viscosity rheograms, which are found from laboratory rheometermeasurements.

In the following the invention will be described in further detail bymeans of an embodiment thereof and the appended drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a flow chart of a process for controlling the spray dropletsize of an spray nozzle apparatus and shows the role of the method ofthe invention;

FIG. 2 is a flow chart of a differential pressure drop method accordingto a specific embodiment of the invention;

FIG. 3 shows a principle of a measuring apparatus for the differentialpressure drop method of the invention

In a preferred embodiment, the method of the invention is carried outwith a product to be delivered to a spray nozzle. Measuring productinput parameters in line with the production process of the powderallows adjusting of the droplet size towards the minimum Sauter diameterpossible inline and thus makes it possible to consider the completerange of spray viscosities during the production process of the powderto be produced.

FIG. 1 is a flowchart of a process for controlling the spray dropletsize of an agglomeration spray nozzle apparatus. The product paste inFIG. 1 indicated as “concentrate” is delivered to a dosing point 30,which leads a part of the product paste stream into a bypass line 32.The majority of the product paste stream is directed into a main productpaste line 34. The bypass line 32 is redirected into the main productpaste line 34 at a line junction 36 downstream of a differentialpressure drop measuring apparatus 38 provided in the bypass line 32.

Downstream of the line junction 36 a mass flow meter 40, a density meter42 and a spray pressure probe 44 are provided in the main product pasteline. Downstream of the spray pressure probe 44 the main product pasteline 34 enters a spray nozzle apparatus 1 through tube 25. The productpaste delivered to the spray nozzle apparatus 1 is then sprayed into aspray drying chamber 46.

The differential pressure drop measuring apparatus 38 determines theshear rate and the shear viscosity η of the product paste delivered tothe spray nozzle, according to one preferred embodiment of theinvention. The data of the shear rate and shear viscosity η aredelivered from the differential pressure drop measuring apparatus 38 toa control device (SPS-control) 48. In the same manner, the product pastemass flow rate Q_(m) determined in the mass flow meter 40, the productpaste density ρ determined in the density meter 42 and the spraypressure P of the product paste determined in the spray pressure probe44 are also delivered to the control device 48. The shear rate has to begreater than 1000 s⁻¹.

Control device 48 comprises a computer which calculates an outputcontrol parameter based on the above data delivered to the controldevice 48 and on the basis of known spray nozzle geometry parametersstored in a memory of the control device 48. The output controlparameter is delivered to the spray nozzle apparatus 1 in order toadjust the swirl chamber piston 17 (plunger) to a calculated position inorder to obtain a desired swirl chamber volume.

The following equations 1-7 describe the solving procedure how tocontrol the plunger position (given with h_(sc)) based on a change inthe paste shear viscosity η.

Accordingly the solving procedure is applied for a change in mass flowrate Qm and paste density ρ.

Universal Massflow-Characterization of Pressure Swirl Nozzle Flows:

$\begin{matrix}{\frac{Qm}{\eta\mspace{11mu} d_{sc}} = {2.1844\left( \frac{d_{or}}{d_{sc}} \right)^{1.2859}\left( \frac{h_{sc}}{d_{sc}} \right)^{0.4611}\left( \frac{\sqrt{P\;\rho}d_{sc}}{\eta} \right)^{0.9140}}} & (1)\end{matrix}$The relation between spray pressure P and axial position of the plunger(given with h_(sc)) is derived for the example of a shear viscositychange from η_(old) to η_(new):

$\begin{matrix}{\frac{\eta_{new}}{\eta_{old}} = {\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{0.4611}\left( \frac{\eta_{new}}{\eta_{old}} \right)^{0.9140}\left( \frac{P_{old}}{P_{new}} \right)^{\frac{0.9140}{2}}}} & (2)\end{matrix}$Solved for the spray pressure ratio:

$\begin{matrix}{\frac{P_{old}}{P_{new}} = {\left( \frac{\eta_{new}}{\eta_{old}} \right)^{\frac{1 - 0.9140}{0.4570}}\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{\frac{- 0.4611}{0.4570}}}} & (3)\end{matrix}$In order to find a direct relation between plunger position h_(sc) andshear viscosity η, the spray pressure ratio has to be found from anotherequation, see equations 4-6 below:

Universal Spray Droplet Size Characterization of Pressure Swirl NozzleSprays:

$\begin{matrix}{\frac{D_{32,{global}}}{d_{sc}} = {1.0798\mspace{14mu}{Re}^{- 0.2987}{{We}^{- 0.1709}\left( \frac{h_{sc}}{d_{sc}} \right)}^{- 0.0772}\left( \frac{d_{or}}{d_{sc}} \right)^{0.9534}}} & (4)\end{matrix}$Again, one can derive the Spray Pressure Ratio with the consistencyconditions that D_(32-global-old) and D_(32-global-new) remain constant:

$\begin{matrix}{\begin{matrix}{\frac{D_{32,{global},{old}}}{D_{32,{global},{new}}} = 1} \\{= {\left( \frac{{Re}_{old}}{{Re}_{new}} \right)^{- 0.2987}\left( \frac{{We}_{old}}{{We}_{new}} \right)^{- 0.1709}\left( \frac{h_{{sc},{old}}}{g} \right)^{- 0.0772}}} \\{= {\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{- 0.2987}\left( \frac{\eta_{old}}{\eta_{new}} \right)^{0.2987}\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{0.2987}}} \\{\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{0.1709 \cdot 2}\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{- 0.0772}}\end{matrix}\quad} & (5)\end{matrix}$And hence the solution, how to control the plunger height h_(sc,new)based on a current position h_(sc,old):

$\begin{matrix}{\frac{h_{{sc},{new}}}{h_{{sc},{old}}} = \left( \frac{\eta_{new}}{\eta_{old}} \right)^{- 1.1289}} & (6)\end{matrix}$Combining equations 3 and 6 one receives the solution, how to controlthe spray pressure:

$\begin{matrix}{\frac{P_{new}}{P_{old}} = \left( \frac{\eta_{new}}{\eta_{old}} \right)^{0.9508}} & (7)\end{matrix}$

FIG. 2 is a flowchart of the differential pressure drop method asapplied in the differential pressure drop measuring apparatus 38 andaccording to a preferred embodiment of the invention. A feed pump 50 isprovided in the bypass line 32 downstream of dosing point 30. The feedpump 50 ensures a constant feed-flow-rate in the differential pressuredrop measuring apparatus 38 to enable shear rates which cover the secondNewtonian viscosity plateau. Downstream of the feed pump 50 a mass flowmeter 52 is provided through which the product paste in the bypass line32 is directed into a pressure drop meter 54. The shear viscosity (η) ofthe product paste in the bypass line 32 is calculated from the mass flowmeasured in the mass flow meter 52, the known product density of theproduct paste and the pressure drop measured in the pressure drop meter54. This calculation is either made in a computer (not shown) of thedifferential pressure drop measuring apparatus 38 or, the respectivedata are delivered to the control device 48 and the shear viscosity η iscalculated in the computer of the control device 48. In order toconsider the fact that the pressure drop is measured in a bypass line 32the bypass mass flowrate is adjusted by the feed pump 50 until theshear-rate is above 1000 s⁻¹, so that the second Newtonian plateauviscosity can be measured by the pressure drop-meter 54 within laminarflow conditions.

A pulsation damper is also preferably provided in the bypass to reducethe noise in the pressure determination.

In the present example the dosing point 30 regulates the bypass flowrate to keep the bypass flow pressure <20 bar at laminar flowconditions, with a Reynolds number below 2300.

FIG. 3 shows the principle of a measuring apparatus (pressure dropmeter) for the differential pressure drop method for determination ofthe second Newtonian plateau viscosity using three independent pressuredrop recordings at three different shear-rates.

The pressure drop meter 100 comprises a tube having a fluid inletsection 102 and a fluid outlet section 104 and three pressure dropmeasuring sections 106, 108, 110 provided between the inlet section 102and the outlet section 104. The first pressure drop measuring section106 which is close to the inlet section 102 has a first internaldiameter d₁ and a first axial length I₁. A first differential pressuremeter 112 measuring a first pressure drop Δp₁ is connected to the firstpressure drop measuring section 106 in a commonly known matter whereinthe axial distance L₁ between the two static pressure measuring openingsin the wall of the first pressure drop measuring section 106 issubstantially equal to the length I₁ of the first pressure dropmeasuring section 106.

The second pressure drop measuring section 108 is provided downstream ofthe first pressure drop measuring section 106. The internal diameter d₂of the second pressure drop measuring section 108 is smaller than thediameter d₁ of the first pressure drop measuring section. The length I₂of the second pressure drop measuring section 108 is shorter than thelength of the first pressure drop measuring section 106. The secondpressure drop measuring section 108 comprises a second differentialpressure meter 114 measuring a second pressure drop Δp₂ wherein thedistance L₂ between the two static pressure measuring openings in thewall of the second pressure drop measuring section 108 is shorter thanthe distance L₁ of the first differential pressure meter 112.

A third pressure drop measuring section 110 is provided downstream ofthe second pressure drop measuring section 108 and the third pressuredrop measuring section 110 opens into the outlet section 104. Theinternal diameter d₃ of the third pressure drop measuring section 110 issmaller than the diameter d₂ of the second pressure drop measuringsection 108 and the length I₃ of the third pressure drop measuringsection is shorter than the length I₂ of the second pressure dropmeasuring section. The third pressure drop measuring section 110comprises in a commonly known manner a third differential pressure meter116 measuring a third pressure drop Δp₃. The distance L₃ between the twostatic pressure measuring openings in the wall of the third pressuredrop measuring section 110 is shorter than the distance L₂ of the seconddifferential pressure meter 114.

The differential pressure drop meter 100 allows the measurement of threeindependent pressure drop recordings of the first, the second and thethird differential pressure drop meters. Utilizing these threedifferential pressure drop probes in series, a single mass flow ratecauses three increasing wall shear rates with the decreasing tubediameter.

The following equation 8 is used to calculate the shear viscosity η forlaminar tube flows (Re<2300), applied to all 3 differential pressuresΔp₁, Δp₂ and Δp₃ (respectively measured at 112, 114 and 116, FIG. 8), byreplacing Δp_(i) and the corresponding tube dimensions (R_(i) and L_(i))in equation 8:

Only, if the shear viscosity η_(i) is equal (η₁=η₂=η₃) between the 3differential pressures, the 2^(nd) Newtonian shear viscosity is foundand used e.g. in equation 1 and 7, etc.

$\begin{matrix}{\eta_{i} = \frac{{\pi \cdot R_{i}^{4} \cdot \Delta}\;{p_{i} \cdot \rho}}{8 \cdot {Qm} \cdot L_{i}}} & (8)\end{matrix}$with following definitions of symbols:

-   R_(i): tube radius (R₁, R₂ and R₃) in [m]-   Δp_(i): tube pressure drop (Δp₁, Δp₂ and Δp₃) in [Pa]-   ρ: product density in [kg/m3]-   Qm: mass flow rate in [kg/s]-   L_(i): tube length (distance L₁, L₂ and L₃) in [m]

TABLE 1 Abbreviations and formula Symbol, Abbreviation Description UnitsD_(32,global) Global Sauter diameter as found [m] from PDA measurementsof spray d_(sc) Swirl chamber diameter [m] (smallest diameter of swirlchamber spiral) h_(sc) Swirl chamber height [m] (axial height of swirlchamber) d_(or) Orifice diameter [m] (diameter of opening made inorifice plate) b_(ch) Width of swirl chamber inlet [m] channel (smallestwidth of inlet channel which leads into the swirl chamber) We$\quad\begin{matrix}{{Weber}\mspace{14mu}{number}} \\{{We} = \frac{\rho_{liquid}u_{bulk}^{2}d_{orifice}}{\sigma_{liquid}}}\end{matrix}$ — Eu $\quad\begin{matrix}{{Euler}\mspace{14mu}{number}} \\{{Eu} = \frac{P}{\rho_{liquid}u_{bulk}^{2}}}\end{matrix}$ — Re $\quad\begin{matrix}{{Reynolds}\mspace{14mu}{number}} \\{{Re} = \frac{\rho_{liquid}u_{bulk}h_{sc}}{\mu}}\end{matrix}$ — u_(bulk) $\quad\begin{matrix}{{Bulk}\mspace{14mu}{velocity}\mspace{14mu}{at}\mspace{14mu}{swirl}\mspace{14mu}{chamber}\mspace{14mu}{inlet}} \\{u_{bulk} = \frac{Qm}{\rho_{liquid}h_{sc}b_{ch}}}\end{matrix}$ [m/s] Qm Mass flow rate [kg/s] P Spray pressure [Pa]ρ_(liquid) Liquid density [kg/m³] η_(liquid) Liquid shear viscosity [Pa· s] σ_(liquid) Surface tension [N/m] PDA Phase-Doppler Anemometry —

The invention should not be regarded as being limited to the embodimentshown and described in the above but various modifications andcombinations of features may be carried out without departing from thescope of the following claims.

The invention claimed is:
 1. A method of continuously determining ashear viscosity of a product paste in a processing line, the methodcomprising: continuously determining the shear viscosity of the productpaste in a bypass to a stream of the product paste, the bypasscomprising a pump, a flow meter, a differential pressure tube and apulsation damper, and the shear viscosity is in a range of 20 to 1000mPa·s, the shear rate is greater than 1000 s⁻¹, and the Reynolds numberis less than 2300; delivering the product paste to a spray nozzle for aspray-drying application; and adjusting a droplet size of the productpaste, the adjusting comprises a control device calculating an outputcontrol parameter based on the shear rate and the shear viscosity of theproduct paste and also based on spray nozzle geometry parameters storedin a memory of the control device, the output control parameter isdelivered to the spray nozzle which adjusts a swirl chamber piston to acalculated position to obtain a desired swirl chamber volume.
 2. Themethod according to claim 1, wherein the determining the shear viscosityof the product paste comprising: a) providing a constant feed-flow-rateof the product paste; b) determining a mass flow of the product paste;c) delivering the product paste to a pressure-drop-meter and determininga pressure drop; d) calculating the shear viscosity of the product pasteon the basis of the mass flow determined in step b), a known productdensity, as well as the pressure drop determined in step c).
 3. Themethod according to claim 2, wherein the calculation in step d)considers also the constant bypass-mass-flow-rate.
 4. The methodaccording to claim 2, wherein the determination of the pressure drop instep c) is carried out according to a differential pressure drop method.5. The method according to claim 2, wherein the pressure drop metercomprises a tube having a fluid inlet section and a fluid outlet sectionand a first, second, and third pressure drop measuring sections providedbetween the inlet section and the outlet section.
 6. The methodaccording to claim 5, wherein the second pressure drop measuring sectionis provided downstream of the first pressure drop measuring section, hasa second internal diameter of the second pressure drop measuring sectionsmaller than the first internal diameter of the first pressure dropmeasuring section, and has a second axial length shorter than the firstaxial length of the first pressure drop measuring section.
 7. The methodaccording to claim 6, wherein the second pressure drop measuring sectioncomprises a second differential pressure meter measuring a secondpressure drop, a second axial distance between second two staticpressure measuring openings in a second wall of the second pressure dropmeasuring section is shorter than the first axial distance of the firstdifferential pressure meter.
 8. The method according to claim 5, whereinthe third pressure drop measuring section is provided downstream of thesecond pressure drop measuring section opens into the outlet section,and the third pressure drop measuring section has a third internaldiameter smaller than the second internal diameter of the secondpressure drop measuring section and has a third axial length shorterthan the second axial length of the second pressure drop measuringsection.
 9. The method according to claim 8, wherein the third pressuredrop measuring section comprises a third differential pressure metermeasuring a third pressure drop, and a third axial distance betweenthird two static pressure measuring openings in a third wall of thethird pressure drop measuring section is shorter than the second axialdistance of the second differential pressure meter.
 10. The methodaccording to claim 5, wherein the first pressure drop measuring sectionis close to the inlet section, has a first internal diameter and a firstaxial length, and is connected to a first differential pressure metermeasuring a first pressure drop.
 11. The method according to claim 10,wherein a first axial distance between first two static pressuremeasuring openings in a first wall of the first pressure drop measuringsection is substantially equal to the first axial length of the firstpressure drop measuring section.